Fossil fuel combustion is used in industrial processes for many different purposes. Unfortunately, fossil fuel combustion produces several contaminants, which have been found to be detrimental to the environment. In particular, sulfur and nitrogen oxide compounds are the major components of “acid rain”. Sulfur is a naturally occurring element in crude oil, concentrated in the residual components of the crude oil distillation process. The amount of sulfur in the fuel oil depends mainly on the source of crude oil, and to a lesser extent on the refining process. Typically for fuel on a worldwide basis, the value is in the order of 1.5-4% w/w. These values lead to high concentration of SO2 in flue gases. For example, when using a fuel containing 1.5% w/w sulfur, the concentration of SO2 in the emitted gas is about 630 ppm, and with a fuel containing 4% w/w sulfur, the concentration of SO2 in the emitted gas is about 1700 ppm.
In recognition of the harm caused by sulfur oxides (SOx) and nitrogen oxides (NOx), different combustion gas cleaning processes and separation technologies have been developed to remove these components of combustion flue gases prior to release of the flue gases into the atmosphere, especially since the burning of fossil fuel releases many millions of tons of SO2 every year.
European waters were the first in the world to introduce more stringent sulfur emission regulations for ships, with the coming into force of so-called Sulfur Emission Control Areas (SECAs) in the Baltic Sea in 2006, followed by the North Sea and English Channel in 2007.
Under the European Union (EU) Marine Sulfur Directive, only low-sulfur fuels of less than 1.5% w/w sulfur are permitted. Furthermore, a 1.5% w/w sulfur cap in fuel applies to fuels used by passenger vessels operating regular services to and from any community port from Aug. 11, 2006, and not just in SECAs. The EU legislation allows the use of technologies that abate the sulfur content in the emitted gas as an alternative to using low-sulfur fuels (of 1.5% w/w sulfur). Thus, the technology should assure reductions in sulfur emissions that are at least equal to, or better, than those achieved by lowering the sulfur content in bunker fuel.
Most of the technologies for reducing the content of SOx in the exhaust gas use wet scrubbing processes in which the exhaust gas is contacted with an aqueous solution. The aim of these processes is to provide high absorption efficiencies, i.e., above 70-95% SO2 absorption. The demands relating to an absorption process are high, leading to relatively few design options. For example, the efficiency of the reaction between the components in the aqueous solution and the SOx in the exhaust gas phase being treated decreases as the reaction temperature increases, especially above about 70° C. However, the temperature of the emitted gas is about 300° C. thereby diminishing the efficiency of the process.
In addition, the volume of the emitted exhaust gas is significant, reaching an output of about 12000 m3/hr for a relatively small engine with a capacity of about 1 MW. Thus a high efficiency process is required in order to avoid the dependence on large equipment to process the large volume of gas. Equipment size is particularly critical on board of ships where the available space is limited.
Purification of exhaust gas from marine vessels by wet scrubbing is well known. US2016016109, for example, discloses a vertical scrubber tower with an upper and lower chamber, each with water injectors, deflection bodies and a scrubber liquid outlet.
EP1857169 discloses a scrubbing system based on fresh water comprising a sulfur removal agent in a packed bed scrubber. Packed beds in scrubbers often generate high backpressure to the engine. This problem can be solved by introducing an exhaust fan after the scrubber. Packed bed scrubbers also require a bypass-system because the packed bed can be damaged during dry running. Furthermore, the packed bed scrubbers are required to have silencer systems and operate with fresh water only.
Scrubbers for power stations comprising a plurality of diesel engines are also known. WO2006048506 for example, discloses how engine exhaust gases from multiple engines are arranged to be conveyed in separate pipes all the way to the scrubber.
It is emphasized that, although scrubber technology suitable for land based installations is already known, these technologies are not always suitable for marine applications since the size, the weight, the robustness, the stability, the flow and the exhaust composition are parameters that are not automatically interchangeable between a marine and a land based scrubber system.
The purpose of the application is to provide a scrubbing system not creating a high backpressure to the engine and at the same time allowing a multiplicity of engine exhaust gas pipes to be connected to the scrubber tower in a simple way and requiring a limited amount of space.